Patients with cystic fibrosis (CF) have an increased risk of urolithiasis/nephrocalcinosis. To determine potential mechanisms responsible, we studied the urinary excretion of lithogenic and stone-inhibitory substances and calculated the urinary saturation for calcium-oxalate (CaOx), brushite (CaHPO4), and uric acid (UA). We examined 24-h urines in 63 patients with CF (34 female, 29 male) aged 5 months to 36 years. Renal ultrasonography was performed at the time of urine collection. Hyperoxaluria was found in 25 patients (range 0.51-1.71 mmol/1.73 m2 per 24 h). Urinary Ca was increased in 13 patients (4.1-8.22 mg/kg per 24 h). Hyperuricosuria was found in 16 patients (5.2-18.0 mmol/1.73 m2 per 24 h) and hypocitraturia in 14 patients (0.07-1.14 mmol/1.73 m2 per 24 h). CaOx saturation was elevated in 26 patients, related to hyperoxaluria in 19 patients. CaHPO4 saturation was increased in 19 patients and UA saturation in 11 patients. Urolithiasis in situ was diagnosed in 1 patient; 3 patients previously had renal stones; 4 patients had present nephrocalcinosis. Elevated excretion of lithogenic substances and urinary supersaturation might lead to the higher risk of urolithiasis/nephrocalcinosis in patients with CF.